Thin-film electroluminescent structures are of great interest because they offer a possible means of achieving a high performance flat panel color display. The search for an efficient blue light-emitting phosphor is intense because a blue light-emitting phosphor having sufficient luminance and efficiency is a prerequisite for the color flat panel display application. In recent years researchers have focused on producing a stable phosphor that is capable of emitting blue light of the required chromaticity and intensity.
Research has previously focused on cerium-doped strontium sulfide and thulium-doped zinc sulfide. Unfortunately, the SrS:Ce phosphors are chemically unstable and have poor color purity, and the ZnS:Tm phosphors demonstrate poor luminance.
Sohn and others have reported that incorporating oxygen into terbium-doped zinc sulfide improved the crystallinity of the ZnS, lowered the threshold voltage of the phosphor and increased phosphor luminance, in "Effects of Oxygen on Electroluminescent Characteristics of ZnS:TbOF and ZnS:TmOF Devices," J. Appl. Phys.72, 4877-83 (1992). Sohn and others report that oxygen doping increased the luminance of cerium-doped zinc sulfide, in "A Model for Emissions from ZnS:Ce.sup.3+ and SrS:Ce.sup.3+ Thin-Film Electroluminescent Devices," Jpn. J. Appl. Phys. 31, 3901-06 (1992).
Investigators have identified rare earth doped alkaline earth thiogallate compounds as efficient blue light and green light-emitting phosphors suitable for use in thin-film electroluminescent panels, as shown, for example, in U.S. Pat. No. 5,309,070 to Sun et al.
The electroluminescent emission spectrum of cerium-doped strontium or calcium thiogallate shows overlapping double bands with two peaks. The emission spectrum of the excited Ce.sup.3+ ion results from transitions from the same 5d excited state to the 4f ground state. Two peaks result because the 4f ground state is a doublet. However, since the 5d excited state of Ce.sup.3+ is not shielded by the 5s.sup.2 5p.sup.6 shells and therefore is easily affected by the crystal field, the transition energy varies with the host materials. The wavelengths of the cerium emission peaks in strontium thiogallate are 445 nm and 490 nm; the emitted light has a deep blue color with CIE coordinates of x=0.15, y=0.10. The wavelengths of the cerium emission peaks in calcium thiogallate are shifted about 15 nm to longer wavelengths due to a stronger crystal field; the emitted light has CIE coordinates of x=0.15, y=0.20. Thus, the color of light emitted by cerium in calcium thiogallate is not as deep a blue as the light emitted by cerium in strontium thiogallate.
The human eye is more sensitive to green light than to blue light. Therefore, the same number of photons (radiative quantity) will produce more luminance (photopic quantity) from cerium-doped calcium thiogallate than from cerium-doped strontium thiogallate. In addition, the luminous efficiency (lumens/watt) of cerium-doped calcium thiogallate is almost twice that of cerium-doped strontium thiogallate. However, when the luminous efficiency is normalized by the spectral luminous efficacy (the ratio of the photopic output to the radiative input) to obtain the radiative efficiency, one finds that the radiative efficiency of cerium-doped calcium thiogallate is not much higher than that of cerium-doped strontium thiogallate.
In a full color electroluminescent display the blue light-emitting phosphor determines the chromaticity and luminous efficiency of the white light when all primary color light-emitting phosphors are emitting. Thus, the performance of the blue light-emitting phosphor must be optimized. The radiative efficiency is a better gauge than the luminous efficiency for evaluating the performance of a blue light-emitting phosphor. The higher the radiative efficiency of the blue light-emitting phosphor, the higher the luminous efficiency of the emitted white light. The improvement in the radiative efficiency of the blue light-emitting phosphor may be due either to an improvement in the emission color to a deeper blue chromaticity, or to an improvement in the brightness, that is, a higher luminous efficiency.
Thus, a need still exists for an efficient blue light-emitting phosphor suitable for use in full color TFEL panels.
According to the present invention, such a need is satisfied by a thin-film electroluminescent (TFEL) structure for emitting light in response to the application of an electric field that includes first and second electrode layers sandwiching a TFEL stack, the stack including first and second insulator layers and a phosphor layer that includes an alkaline earth thiogallate doped with oxygen.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.